What causes ringing in one ear only?

Tinnitus: Asymmetric tinnitus is an acoustic neuroma until proven otherwise. Your ENT must examine you and evaluate with an audiogram and MRI if suspicious of neuroma. Otherwise it may be due to ear infection or acoustic trauma or loud noise exposure in that ear. Almost any other etiology would be bilateral. meniere's can also theoretically cause unilateral tinnitus but usually with hearing loss or fullness